In this context, DE 10 2008 018 390 A1 discloses the synchronous rectifiers illustrated in FIGS. 1 and 2, in which a plurality of the type of rectifier cells Gi illustrated in FIG. 3 are used. FIG. 1 shows a synchronous rectifier constructed as a bridge rectifier, while FIG. 2 shows a synchronous rectifier constructed as a push-pull rectifier. For details concerning the structures illustrated in FIGS. 1 to 3, reference is made to the aforementioned DE 10 2008 018 390 A1. As indicated in FIG. 3, the winding sense of the secondary winding of the respective rectifier cell Gi can vary, as can the circumstance of which of the two terminals is used as input and which as output of the rectifier cell, depending on which synchronous rectifier is constructed and depending on the position of the rectifier cell Gi within the respective synchronous rectifier. Therefore, one terminal of the rectifier cell Gi is designated by GE1 or GA1, and the other is correspondingly designated respectively by GA1 or GE1.
In the case of the push-pull rectifier illustrated in FIG. 2, the inductances L1, L2, L3 form a center tapped transformer Tr2, while the inductances L12, L13 arranged in series with the secondary windings L2, L3 of the transformer Tr2 are the primary windings of a control transformer Tr1, the secondary windings of which are arranged in the respective rectifier cells Gi. In comparison with the scenarios known from the paper “A New Synchronous Rectifier Using Bipolar Transistor Driven by Current Transformer” by Sakai, E. and Harada, K., published in 14th International Telecommunications Energy Conference 1992, INTELEC '92, Oct. 4-8, 1992, pages 424-429, in the case of the known scenarios illustrated in FIGS. 1 to 3, the bipolar transistors are turned off in forward operation and are operated in the on state in inverse operation. The range of operating voltages with which such synchronous rectifiers can be operated can be significantly increased as a result. With regard to the further prior art, reference is made to the paper “Improving Efficiency of Synchronous Rectification by Analysis of the MOSFET Power Loss Mechanism”, Infineon Application Note AN 2012-03 V2.1 March 2012, the paper “Control Driven Synchronous Rectifier in Phase Shifted Full Bridge Converters”, Texas Instruments Application Note SLUA 287, and the paper “A novel high efficient approach to input bridges” by Davide Giacomini and Luigi Chine, presented at the PCIM 2008.
If the rectifier cell Gi illustrated in FIG. 3 is used in the synchronous rectifiers in accordance with FIG. 1 and FIG. 2, then in practice at operating frequencies starting from approximately 20 kHz the resulting efficiency has been unsatisfactory. However, the operation of such synchronous rectifiers with the highest possible frequencies is desirable in order to keep the magnetic components small.
US 2002/0110005 A1 discloses a power converter that uses synchronous rectifiers. FIG. 4 of the aforementioned document illustrates a rectifier cell including a first and a second main switch. A first auxiliary switch is assigned to the first main switch and a second auxiliary switch is assigned to the second main switch. A first capacitor is coupled to a first auxiliary winding, and a second capacitor is coupled to a second auxiliary winding. In order to provide a discharge path for the gate terminals of the main switches when the latter are in the off state, the gate terminal of the first main switch is coupled to that terminal of the first capacitor which is not coupled to the first auxiliary winding, wherein the gate terminal of the second auxiliary switch is coupled to that terminal of the second capacitor which is not coupled to the second auxiliary winding. The source-drain path of the first auxiliary switch is coupled between that terminal of the first capacitor which is not coupled to the first auxiliary winding and the reference potential. In this way, the gate terminal of the first main switch can be discharged by the first auxiliary switch being turned on. The same correspondingly applies with regard to the second auxiliary switch and the second main switch.